Intellectually disabled boys abused at an Adelaide Catholic school were the worst-affected victims ever seen by an experienced therapist.

An intellectually disabled boy became so angry at being repeatedly sexually abused by his school’s bus driver that he put a knife to the man’s face, an inquiry has been told.

“I wanted to kill him, I wanted him to die,” the now 38-year-old man told the Royal Commission into Institutional Responses to Child Sexual Abuse on Monday, at its first Adelaide sitting.

The commission’s ninth case study is focused on events from 1986 to 1991 at St Ann’s Special School for children with intellectual disabilities.

It will investigate responses made by the South Australian police, the school and the Catholic Archdiocese of Adelaide to claims of child sexual abuse by Brian Perkins.

Counsel assisting Sophie David said Perkins had three child sex convictions when he became the school’s bus driver after the principal failed to carry out a police check.

“He also volunteered in the woodwork class and provided respite care to parents on weekends,” she said.

In 1991, he was investigated over pornographic photos of students, but after his arrest he skipped bail in 1994 and was not extradited until 2002 despite police knowing his whereabouts earlier.

In 2003 he pleaded guilty to sex charges and was sentenced to 10 years jail, where he died in 2009.

Malcolm Robinson, a social worker and therapist with 43 years experience, said he counselled nine victims.

“I am not an overly emotive person, by nature and training,” he said.

“However, the degree and impact of the abuse in relation to former St Ann’s students I have worked with has been among the worst I have seen in my career.”

The former student, known as LH, said Perkins abused him in the bus, in the school’s woodwork shed and at weekend sleepovers.

He and other students would be molested and another man would photograph them naked.

Perkins once took him to another house where LH’s hands and legs were tied, string was put around his and Perkins’s genitals and a man smacked them.

The mother of another student, who has Downs Syndrome, broke down while telling of the change in her son after he was abused by Perkins.

“He even distrusted Santa Claus when he previously loved Christmas and Santa Claus,” she said.

She often queried St Ann’s staff about why the school bus was regularly late in dropping off her son and was told the vehicle had broken down or had mechanical problems or children were being disruptive.

On three occasions, Perkins asked if her son could attend sleepovers at the school or go on a camp, requests she agreed to as he seemed very convincing and trustworthy and was “so loved by the school”.

In 1991, the school asked her to a meeting where she was informed police suspected Perkins had molested her son.

But, despite repeated requests in the ensuing years, neither police nor the school gave her any further information.

More than 10 years later, her husband had to identify their son in pornographic photos, although she believed police had the pictures for a decade.

The hearing is continuing.